Hints of dark matter at the bottom of a mine

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POSTED: Friday, December 18, 2009

An international team of physicists working in the bottom of an old iron mine in Minnesota said yesterday that they might have registered the first faint hints of a ghostly sea of subatomic particles known as dark matter long thought to permeate the cosmos.

The particles showed as two tiny pulses of heat deposited over the course of two years in chunks of germanium and silicon that had been cooled to a temperature near absolute zero. But, the scientists said, there was more than a 20 percent chance that the pulses were caused by fluctuations in the background radioactivity of their cavern, so the results were tantalizing, but not definitive.

Gordon Kane, a physicist from the University of Michigan, called the results “;inconclusive, sadly,”; adding, “;It seems likely it is dark matter detection, but no proof.”;

Kane said results from bigger and thus more sensitive experiments would be available in a couple of months.

The team, known as the Cryogenic Dark Matter Search, announced its results in a pair of simultaneous talks by Jodi Cooley from Southern Methodist University at the SLAC National Acceleratory Laboratory in California and by Lauren Hsu of the Fermi National Accelerator Laboratory in Illinois at Fermilab, and they say they plan to post a paper on the Internet.

The stakes for astronomy and physics could hardly be greater. If the particles are confirmed by tests at other detectors, it would mean that, after more than half a century of speculation, astronomers are zeroing in on the identity of the invisible material that accounts for 25 percent of the universe and determines the architecture of the visible universe.

Confirmation of the particles would also constitute the first evidence for a new feature of nature, called supersymmetry, that physicists have been seeking as avidly as the astronomers have been seeking dark matter. It is central to theoretical efforts like string theory, which unify all of the forces of nature into one mathematical expression.

The report ended weeks of speculation on physics blogs and in laboratory cafeterias around the world. At the Kavli Institute for Theoretical Physics in Santa Barbara, Calif., where dark matter experts who had gathered for a two-week workshop watched the talks on the Web, Kane, who was present, described the mood at the workshop as “;a high level of serious hysteria.”;

Dark matter became a serious issue in the 1970s, when Vera Rubin of the Carnegie Institution of Washington and her colleagues charted the rotation speeds of galaxies and found that they seemed to be enveloped in halos of dark matter, then called missing mass.

A wide range of astrophysical and cosmological measurements have subsequently converged on an intimidating recipe for the cosmos of 4 percent atoms, 25 percent dark matter and 70 percent a mysterious energy that has been called dark energy and has nothing to do with the news on Thursday.

The cryogenic experiment is nearly half a mile underground in an old iron mine in Soudan, Minn., to shield it from cosmic rays. It consists of a stack of germanium and silicon detectors, cooled to one-hundredth of a degree Kelvin. When a particle hits one of the detectors, it produces an electrical charge and deposits a small bit of energy in the form of heat, each of which are independently measured.

By comparing the amounts of charge and heat left behind, the collaboration's physicists can tell so-called wimps from more mundane particles like neutrons, which are expected to flood the underground chamber from radioactivity in the rocks around it.

The team is planning a larger detector, called SuperCDMS. In the meantime, Elena Aprile of Columbia, who was also present in Santa Barbara, said the results would be tested soon by her own detector, called Xenon, filled with liquid xenon, which just began working this fall under the Alps in Italy.

“;All eyes will be on Xenon,”; she said in an interview a few days before, explaining that her detector, which is bigger, should see more events, adding, “;Otherwise there will be a big clash.”;